Interesting results for open Riemann surfaces As far as I know, interesting results for open Riemann surfaces are quite rare. One of them is the theorem of Gunning and Narasimhan, which asserts that every connected open Riemann surface admits a holomorphic immersion into the complex plane. Another example is given by the theorem of Behnke and Stein, which says that every connected open Riemann surface is a Stein manifold. Is there any more interesting results for open Riemann surfaces?
 A: For example, a theorem of Grauert and Röhrl asserts that every holomorphic vector bundle on a non-compact Riemann surface is trivial.
You can find this result (and its proof) in the book of O. Forster, Lectures on Riemann Surfaces. 
A: The Riemann uniformization theorem.
A: I hope that the following result by Bishop and myself might be "interesting": Every open Riemann surface is a cover over the sphere, branched over only three points. Equivalently, every open Riemann surface is equilaterally triangulable.
See
Belyi functions on non-compact surfaces; or: Building Riemann surfaces from equilateral triangles and https://arxiv.org/abs/2103.16702 .
A: The results on open Riemann surfaces are not "rare". They are just well forgotten.
I only list a few books which deal with open Riemann surfaces:
MR0114911, MR0228671, MR0159935, MR0264064, MR1973182.
It is true that there are "too many" Riemann surfaces, and not too much can be said about
"all of them". However, there is a highly non-trivial classification, and some subclasses
are very important. For example, the class of "hyperelliptic" surfaces of infinite
genus plays a very important role in the study of Schrodinger operators and their
finite difference analog, see for example the works of Sodin and Yuditskii, MR1288838,
and Kean, Moerbeke, MR0397076. Abelian covers of compact surfaces is another class
of open Riemann surfaces that was studied a lot: MR0740581.
